Yup,that would be truly tough to advise or predict the possible problem unless we have some more details on this regard.There are multiple reasons that could stop outgoing e-mails from an account.
As an SOS,you may use the troubleshooter as suggested by MikeTrike. Still if the problem persists,you can anytime come back to the forum with some more details such as the control panel that the client is using,the mail client etc.